Newport Beach city officials recently ordered a real instate investor to stop gutting the interior of an office building designed by celebrated modernist architect Richard Neutra.

It is the latest dispute concerning the Mariners Medical Arts building, a sleek 1963 complex at 1901 Westcliff Drive saved from demolition in 2009.

Preservationist John Linnert, a Costa Mesa architect, noticed crews working on the upstairs interior in January and reported them to the city planning staff. He has kept a close watch on the building in recent years.

Newport officials issued a stop-work order, as the building owner had no permits.

"He gutted the whole upstairs of the building," said Linnert, who pushed for the building's historical resource designation three years ago. "It's an abomination."

The building's owner, John Bral of Westcliff Investors LLC, did not return calls seeking comment.

Since the city red-tagged his building in mid-January, Bral has applied for permits.

State law prohibits major alterations or demolition at such buildings when the changes threaten the buildings' historic qualities. City Planner Jaime Murillo said he did not know if the interior remodel would trigger that protection.

He added that the Building Division will consult with the city's contract architectural historian. That consultant found in 2009 that the building was eligible for a historic listing at the national, state and local levels.

Neutra was primarily known for his geometric but airy houses, many of which were built in Southern California. He also designed landmark buildings, including the Tower of Hope next to the Crystal Cathedral and the Los Angeles County Hall of Records. He died in 1970.

Almost all of his commercial buildings have been substantially altered over the years, according to a website maintained by Neutra's son Dion, who has supported the preservation of the Mariners building.

It appears that Bral was preparing the office space for a new tenant.

Often, when tenants sign a lease they request a major overhaul. Regardless of the historical designation, so-called tenant improvements usually require city approval. Bral also replaced a gas line without acquiring permits, and the city has required him to apply retroactively.

"The building is such an attribute to the culture of Orange County and Southern California," said Linnert, who has closely chronicled the changes.

In 2009, he and a few other preservationists pressured city officials to halt demolition and study the building's historical significance. They prevailed, and have scrutinized the property ever since.

City planners are now preparing an environmental impact report to evaluate Bral's broader plans for the 20,000-square-foot complex. As an alternative to demolition, he has applied to build a two-story office building that would wrap around part of the structure, increasing the size of complex by about 65%.

In recent years, Bral has torn down an exterior stairway and replaced some exterior light fixtures that followed the building's linear lines. A representative said in 2010 that he was making routine repairs and didn't realize they needed to be completed a certain way.